Engines may use various forms of fuel delivery to provide a desired amount of fuel for combustion in each cylinder. One type of fuel delivery uses a port injector for each cylinder to deliver fuel to respective cylinders. Still another type of fuel delivery uses a direct injector for each cylinder.
Further, engines have also been described using more than one injector to provide fuel to a single cylinder in an attempt to improve engine performance. Specifically, in US 2005/0155578 an engine is described using a port fuel injector and a direct injector in each cylinder of the engine.
Another approach utilizing multiple injection locations for different fuel types is described in the papers titled “Calculations of Knock Suppression in Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection” and “Direct Injection Ethanol Boosted Gasoline Engine: Biofuel Leveraging for Cost Effective Reduction of Oil Dependence and CO2 Emissions” by Heywood et al. Specifically, the Heywood et al. papers describes directly injecting ethanol to improve charge cooling effects, while relying on port injected gasoline for providing the majority of combusted fuel over a drive cycle.
However, the inventors herein have recognized a problem with such approaches when attempting to perform feedback air-fuel ratio control. Specifically, while variation of direct fuel injection may provide a faster response under some conditions, direct fuel injection may not be used under all conditions. Further, the variation caused by feedback adjustments may negatively impact engine combustion and mixture formation, especially if the amount of directly injected fuel is relatively small compared to port injected fuel. Likewise, while variation of port injected fuel based on feedback may be used to provide improved operation under some conditions, again, port injection may not be used under all conditions. Furthermore, variation of port injected fuel under some conditions may result in transient fuel delays and calculation errors.
As such, in one example, a method is provided for operating an engine having a first injector for injecting a first fuel into a cylinder of the engine and a second injector for injection a second fuel into said cylinder of the engine, the engine further having at least an exhaust gas oxygen sensor. The method comprises: varying an amount of said first fuel injection in response to said sensor under a first operating condition; and varying an amount of said second fuel injection in response to said sensor under a second operating condition.
In this way, it is possible to provide feedback variation using an injector most appropriate for the current operating conditions. For example, different fuel injections may provide different transient responses, or may result in different effects on the ratio of exhaust gas constituents at a given speed/load region. Further, variation in different fuel types can have differing effects on engine combustion phenomena, such as knock, pre-ignition, peak combustion temperature, etc.